Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR CONVERTING WEB OR SHEET MATERIAL
AND PRESS FOR APPLYING SAID METHOD
The present invention relates to a method for converting web or
sheet material by compression between an upper table and a lower table, and
to a converting press for applying the method.
Such presses are known particularly in the field of paper or board
converting for making things such as folding boxes.
One kind of press, well known to those skilled in the art, operates
on the paper or board material by converting it flat between two platens, one
fixed and the other movable as described for example in EP0681892. The fixed
platen is an upper table connected to the machine frame. For cutting presses,
the cutting tool mounted on this fixed platen is a plate provided with a
multitude
of knives for cutting and compressing the material to a desired shape. This
plate is generally known to those skilled in the art as a cutting form. The
movable platen is provided with opposing elements acting as an anvil and with
negative indentations for the compressing knives or knife lines. The movable
platen itself takes the form of a lower table connected to a supporting
structure.
In the version described in EP0681892, the lower table is moved vertically in
a
cyclical manner. In other known versions, the lower table is fixed while the
upper table is moved vertically in a cyclical manner. This periodic movement
has the effect of compressing the material processed between the two platens,
thus cutting and/or compressing it in a brief operation covering its entire
surface.
The flat cutting of an area equivalent to that of a cutting platen
requires the use of considerable compressive force. If the knives of the
cutting
form are not to be rapidly blunted, care must also be taken to ensure that the
knives do not press more than is necessary against the anvil as they cut the
material sandwiched between the two platens. Depending on the thickness and
type of material converted, different settings are required, including one
setting
to refine the quality of the cutting by adjusting the height of the knives in
areas
where the material has not been sufficiently well cut. For this purpose the
machine operator places a depth-adjusting sheet on the back of the knives of
the cutting form. This depth-adjusting sheet is designed to have small pieces
of
adhesive tape stuck to the above-mentioned areas. In this way, when the
material is compressed between the two tables, converting of the material will
be more satisfactory because of the variation in height of the cutting edge of
the
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knives relative to the sole plate of the upper table.
One of the disadvantages of this practice is that this depth-
adjusting operation is relatively time-consuming and requires several
successive trials before a satisfactory result is achieved. The time spent on
this
operation is currently the longest part of the operation of preparing the
machine
for a new job. For short runs, this is a non-negligible amount of time and
therefore tends to increase production costs.
A second type of press known at the present time rotary converts
the stock between two cylinders pressed against each other. The upper cylinder
generally comprises a circular cutting tool, which has a development that
corresponds to the selected shape to be cut. The lower cylinder is a smooth
cylinder that acts as a cylindrical anvil. Such a machine is illustrated for
example in EP1331054.
A third type of press uses a flat upper table and a movable lower
table of curvilinear shape. Because of the rounded surface of the movable
table, cutting of the material takes place progressively by movement of the
lower table by its rolling against the cutting plane of the fixed upper table.
Such
a curvilinear cutting press is described in GB914637.
Although this latter method of cutting has the advantage of reducing
the force necessary to convert the material, it has been observed that the
knives of the cutting form which is attached to the upper platen wear rapidly
and
unevenly. Inspection has shown that knives situated in the center of the
cutting
form become blunter much faster than knives at the upstream and downstream
ends of this tool. This anomaly is essentially due to the method of
curvilinear
cutting in which the forces employed are twice as great in the center of the
platen as at its ends supported by the movable structure of the lower table.
These problems require frequent replacement of the knives, cause production
of poorer-quality products and require numerous machine stoppages while the
cutting form is reconditioned. For specialists in the area, these are the main
reasons which prevented the development of curvilinear converting presses,
which have never worked satisfactorily, unlike the other two types of presses
that use flat cutting and rotary cutting.
It is an object of the present invention to solve, at least in part, the
above-mentioned problems so that on the one hand it will no longer be
necessary to use a depth-adjusting operation, and on the other hand the
necessary force applied to the converting tools will be as constant as
possible
to avoid premature deterioration of these tools.
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The invention comprises method for curvilinear converting of web or
sheet material by compression of the material between an upper table and a
lower table. The tables are each connected to a respective supporting
structure
and operate conjointly on either side of the plane of travel of the material
along
a longitudinal axis. During the compression phase, there is a phase of
essentially unidirectional, intentional, elastic deformation of a structure of
one of
the tables. An apparatus for the converting includes the structure described
above. The deformation increases and then decreases as the curvilinear
surface of one table rolls past the other table, for providing generally more
uniform compression as the surface rolls.
A clearer understanding of the invention will be gained from a study
of a preferred but in no way restrictive embodiment illustrated by the
accompanying figures, in which:
Figure 1 is a diagrammatic side view of one embodiment of a press
according to the invention;
Figure 2 is a diagrammatic perspective view of an upper table of a
press seen from above;
Figure 3 is a diagrammatic perspective view of a lower table of a
press seen from below;
Figure 4 is a diagrammatic longitudinal section through a table
formed by a structure in accordance with a second embodiment; and
Figure 5 is a schematic representation in the form of three
diagrams illustrating respectively the stiffnesses in the supporting structure
of a
nondeformable table, the stiffness in an intentionally deformable table, and
the
combination of these stiffnesses as they occur in the press of the present
invention.
From a terminological point of view, and to avoid any confusion in
the following description, the terms upstream and downstream are defined with
reference to the direction of movement of the web or sheet material, as
illustrated by an arrow D in the figures. This material moves from upstream to
downstream following the main axis of the machine in a movement marked by
periodic stops. The terms longitudinal and transverse are defined with
reference
to the main axis of the machine. Furthermore, in order not to overburden the
description by mentioning details of construction that have no direct
relevance
to the invention and are well known to those skilled in the art, the terms
upper
table and lower table denote all those elements situated on the respective
side
of the material and which cooperate to convert the material.
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Figure 1 shows a press 1 for processing a web or sheet material 2
traveling in the direction of movement illustrated by the arrow D. On the
respective sides of the plane of travel of longitudinal axis X, there are an
upper
table 10 and a lower table 20. Each of these tables is connected to a
respective
supporting structure 11 and 21. The supporting structure of the upper table 10
is the frame of the press 1, while that of the lower table 20 is comprised of
the
members which support and move the lower table relative to a cross member
30, which acts as a pedestal or base for the press.
The fixed upper table is arranged a short distance above the plane
along which the material 2 advances. It is equipped with a converting tool 3,
which is the cutting form in the case of a press designed for cutting and
compressing the processed material. Arranged on this cutting form are a
plurality of converting members 5 for the material 2, which are thus made
integral with the upper table, more generally one of the tables 10, 20, or at
least
one of them. The converting members each extend transversely across the
table and are spaced apart along the direction D. Although two converting
members 5 are illustrated in Figure 1, use of more is also contemplated.
Typically, such converting members 5 are, for example, cutting and/or
compressing knives arranged between rubber strips or other strips of resilient
material 4. The purpose of these strips is both to protect the knives and to
provide a surface which contacts and immobilizes the material when the
material is squeezed between the two tables. The line running transversely
level with the most downstream knife (with reference to the direction of
movement D of the material) is termed the first knife line f1. Conversely, the
line
running transversely level with the last or most upstream knife is termed the
last
knife line f2.
The supporting structure 21, formed for example by cams 22 and
rollers 26, imparts to the lower table 20 a relatively complex vertical and
scything movement, part of which causes its work surface 25 to roll with a
pressing action against the upper table 10. The material and the cutting form
are sandwiched between these two tables and the cutting knife lines over which
the work surface rolls cut through the material. The direction of rotation of
the
movable table is indicated in Figure 1 by the arrow R, which during
compression of the material 2 pivots preferably from the downstream end to the
upstream end, and from the upstream end to the downstream end when the
movable table is lowered to allow the processed material to move forward
briefly again.
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The alternating tilting movement of the work surface 25 is illustrated
in this same figure by the dot-dash lines 25'. This movement cuts the material
2
progressively without distorting it. The technique of curvilinear cutting
allows
much smaller forces to be used than are required to achieve the same result in
a machine designed for flat cutting. In addition, the pressure exerted is
generally smaller. This is because a machine employing the flat cutting
technique must exert a minimum amount of pressure at all cutting or
compressing points across the entire area of the material to be processed. An
increased average pressure is therefore applied in order to make certain that
the minimum pressure is being applied at all points, which of course increases
the forces involved. In the curvilinear cutting technique the work surface is
rolled over the cutting knife lines, so the cutting area at any given instant
is
smaller and the forces involved are thus considerably reduced.
A converting press applying the method according to the invention
can therefore be substantially lighter than the existing machines that use a
flat
cutting and compressing process. The drive system and the frame can
therefore be redesigned and made smaller than those of a flat converting
machine. A press according to the invention is therefore less expensive to
produce, ship and install.
The curvilinear cutting and compressing converting presses found
in the prior art are all based on the same principle as flat converting
presses. In
these presses the frame and tables are therefore designed to be as rigid as
possible so that they suffer the least possible amount of deformation during
the
converting process.
Figure 5 gives an illustration, in the form of diagrams, of the notions
of forces, stiffnesses and deformations that occur in the supporting structure
11
and in the upper table 10, depending on the position of the force F generated
by the compression of the lower table 20 against the upper table. This figure
helps in particular to explain why in known curvilinear cutting machines of
the
prior art the converting tools suffer abnormal wear.
The diagrammatic representation of this figure is based on
modeling the elastic resistances calculated for a curvilinear cutting press
such
as that described with reference to Figure 1. Because of the curvilinear
movement of the movable table, the force F exerted by this table therefore
travels from the downstream end to the upstream end along the longitudinal
axis X during the time it takes to convert the material.
The diagram illustrated on the left-hand side of Figure 5 represents
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an upper table of the prior art in the form of a rigid beam connected at its
ends
to its supporting structure, that is to the press frame. Though theoretically
not
flexible, this frame unavoidably has a certain intrinsic elasticity when
subjected
to a large stress. Behaving like a spring, it therefore possesses a certain
intrinsic stiffness Kfrarne. This stiffness is exactly like the constant with
a spring
which is determined by the ratio of the increase in the applied force to the
resulting elongation. In our case, the elongation corresponds to the
deformation
of the frame under the action of the force applied to the frame, as compared
with its rest condition corresponding to when the lower table is withdrawn.
Staying with the left-hand graph which shows the variation of the
stiffness Kp~ame as a function of the position of the force F along the
longitudinal
axis X, it will be seen that this stiffness increases progressively as soon as
the
lower table comes into action to cut the material. This progression continues
until it reaches a maximum halfway through its curvilinear course along the
upper table. The stiffness then declines progressively in the same way as it
had
increased, until it reaches its initial value. Because the resultant of the
two
stiffnesses when placed parallel corresponds to the sum of the stiffnesses,
the
maximum value is therefore twice the initial value. The corresponding force
applied to knives situated halfway between the first knife line f1 and the
last
knife line f2 is also therefore twice that applied at the ends of the cutting
form.
This is why the knives of tools fitted to machines of the prior art wore not
only
too rapidly but also unevenly.
To solve this problem the converting method according to the
present invention comprises a phase of intentional and essentially
unidirectional
elastic deformation of a structure belonging to at least one of the tables,
during
the phase of compression of the material between the tables. These
deformations are advantageously oriented essentially perpendicularly to the
work surface. It should be noted that the intentional nature of these
deformations is clearly aimed at differentiating them from involuntary
deformations such as occur within the same structure in the transverse or
longitudinal directions in particular. Hence, the use of the adverb
"essentially" is
intended to make it clear that the deformations that occur are almost entirely
unidirectional.
In terms of forces, deformation and stiffnesses, the converting
method according to the invention is illustrated in the next two diagrams of
Figure 5. The middle diagram shows the reaction of an upper table 10 of a
converting press according to the present invention, connected at each end to
a
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rigid supporting structure.
In this example the upper table 10 is capable of experiencing
intentional elastic deformations essentially perpendicular to the work
surface,
because of the flexibility of its structure. Its intrinsic stiffness K will
therefore
vary between its ends along the longitudinal axis X. The stiffness, or elastic
resistance, of the upper table 10 will be greater at its ends than in an
intermediate position between these ends. By careful design of the structure
12
of the upper table 10, the variation of its stiffness can be made to almost
cancel
out the increase in stiffness seen in the diagram on the left, giving an
almost
constant final stiffness as shown in the last diagram on the right-hand side
of
Figure 5. The force applied to the converting members 5 of such a press 1 does
not therefore undergo large fluctuations, but rather is as constant as it can
be
during the converting of the material.
Computer modeling has made it possible to compare the forces
along a series of knives in a cutting form fitted first to an ordinary platen
press,
and then to a press according to the invention. The results show that the
ratio
of the minimum force to the maximum force found at all measured points in a
curvilinear cutting press according to the present invention is at least five
times
better than when cutting with an ordinary press. This demonstrates the great
improvement in terms of evenness of the converting force across the whole of
the area of the cutting form.
From a practical point of view, it should be observed that the
stiffness compensation has the result of making the force per unit length of
the
converting members 5, particularly the knives, practically constant as the
movable table rolls against the fixed table. The near constancy of this force
is of
real interest only in the range situated between the first knife line f1 and
the last
knife line f2. It would also therefore be quite adequate to obtain effective
compensation in this region only.
Figure 2 shows an embodiment of the structure 12 of the upper
table 10 seen in perspective in a top view. Notice that this structure is
elastic
and anisotropic so that it only allows itself to be deformed along the
longitudinal
axis X, in a direction perpendicular to the work surface.
Thus, transverse deformations will be kept as low as possible by
reinforcing members 13 in this structure, and particularly by the way in which
they are positioned relative to the longitudinal axis X of travel.
Advantageously, these reinforcing members are on the one hand
laid in an essentially transverse orientation relative to this axis, and on
the other
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hand placed on edge, against a sole plate 14, and thus have the greatest
possible resistance to bending in this orientation. The reinforcing members
preferably consist of ribs whose ends are connected to the side walls of the
frame of the press, either directly, or indirectly via side plates 15. While
on this
subject, notice that the connection may be achieved in some other way and
that, for example, it is not necessary for the entire height of the rib to be
connected.
Thus, it will be observed in Figure 2 that the thicknesses, heights,
shapes or outlines of the reinforcing members 13 can vary so as to influence
the bending of the upper table 10 as a function of the position, along the
longitudinal axis X, of the force applied by the lower table 20. The spacing
between these reinforcing members is also an influencing factor, as is an
optional deliberate non-perpendicularity with respect to the longitudinal axis
of
travel of the material. In a similar way, note that the connection or
attachment of
the edge of the reinforcing member 13 to the sole plate 14 may also have a
particular shape in order to influence the behavior of the upper table when it
is
bent. As illustrated in Figure 2, this sole plate is preferably not directly
attached
to the side plates 15. In order to permit only bending of the table in the
longitudinal direction, the structure 12 is advantageously neither closed, nor
provided with cross ribs deliberately intended to prevent this bending.
Referring to Figure 3, this figure shows an embodiment of the
structure 22 of the lower table 20, shown in perspective in a view from
beneath.
This embodiment and its characteristics are similar to those described above
with reference to the upper table 10. Thus, this structure also has the same
members as those of the structure 21, namely reinforcing members 23, a sole
plate 24 and side plates 25, the exception being that because of the mobility
of
the lower table 20, the side plates will not of course be connected to the
frame
of the press 1.
Figure 4 shows, in a longitudinal section, a second embodiment of
the structure 12 of one of the tables, in particular of the upper table 10.
Unlike
that described in the previous embodiment, this structure is not necessarily
open and contains no reinforcing member of the rib type. In this structure the
desired elastic deformations are obtained by fitting to the table an elastic
sole
plate 6 of variable stiffness. This variable stiffness or elastic resistance
can be
produced by varying the thickness of the elastic sole plate 6 in a selected
direction, namely essentially in the direction of the longitudinal axis X. The
converting tool 3 is preferably arranged either directly against the elastic
sole
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plate 6 in order to benefit from its intentional deformations, or through an
intermediate bearing plate 7 capable of withstanding the aforesaid
deformations
without ever going outside of the elastic range of the material of which it is
made.
In general terms, it will be pointed out that the structure 12, 22 of
either or both of the tables 10, 20 of the press 1 is comprised of at least
one
variable-stiffness member and that this member can be the elastic sole plate
6,
for example, though it may also be the reinforcing members 13, 23 as
described earlier.
In much the same way, the same reasoning can be applied to the
intrinsic stiffnesses employed in the lower table 20. In general terms, it is
pointed out that the structures of the lower and upper tables are so designed
that the sum of the intrinsic stiffnesses in these tables and in their
supporting
structures 11, 12 is such that it is close to a value which remains constant
as
the movable table rolls against the fixed table. In one particular embodiment,
the two tables 10, 20 each possess an intentionally deformable elastic
structure
and undergo combined intentional deformations.
The radius of curvature at all points of the curvilinear work surface
is preferably greater than or equal to five times the working distance between
the first and last knife lines, thus smoothing out the variations of the load
on the
compressed material and increasing the length of the compressed zone.
In more general terms, it will also be pointed out that the tasks
performed by these presses are not indeed limited merely to cutting and
compressing operations but could be accommodated to any other converting
operation, such as embossing, applying metallized bands or printing.
Advantageously, the present invention makes it possible to
dispense completely with the depth-adjusting operation necessary in ordinary
flat converting presses. This both reduces machine preparation time and allows
these tasks to be done by less skilled staff.
Also advantageously, the present invention makes it possible to
gain the maximum benefit from the advantages of curvilinear converting by
allowing the use of lighter presses thereby reducing costs and environmental
nuisance, while avoiding premature tool wear. As a result, not only is tool
life
extended, but also the quality of cutting and scoring by compression of the
material are improved by comparison with the output of ordinary platen
presses.
